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Horiuchi Y, Hagiwara M, Okamoto K, Kobayashi T, Endo M, Kobayashi T, Sakurada S. Effect of pre-aging treatment on the microstructure and magnetic properties of Sm(Co, Fe, Cu, Zr) 7.8 sintered magnets. Mater Trans. 2014;55(3):482. Tang W, Zhang Y, Hadjipanayis GC. Effect of Zr on the microstructure and magnetic properties of Sm(Co balFe 0.1Cu 0.088Zr x) 8.5 magnets. J Appl Phys. 2000;87(1):399. Beketov VN, Moskalev VN, Taranov DV, Ogurtsov AV, Sharin MK, Popov AG, Terent’ev PB. Structure and properties of Sm–Co–Fe–Cu–Zr magnets for high-temperature applications. Met Sci Heat Treat. 2018;60(7–8):498. Huang G, Song XY, Liu D, Wang DX, Wang HB, Liu XM. Effects of Hf on phase structure and magnetic performance of nanocrystalline SmCo 7-type alloy. J Mater Sci. 2016;51(7):3390. Individual ions with fixed magnetic moments may cooperatively align their moments, resulting in the presence of magnetic properties of the crystal as a whole. Ferromagnet crystals have the magnetic moments from all their constituent ions aligned in the same direction; the magnetic moment of the crystal is the summation of the individual moments of the ions. There must be a magnetic force between the different ions that causes them to cooperatively align their moments. This force is also due to electron exchange. The d-orbitals from neighbouring ions overlap weakly into covalent bonds. The d-electrons on the separate ions are shared with the neighbour through covalent bonding. The electron exchange will tend to align the spins on the two neighbours. Aligning all pairs of neighbours aligns all ions. The exchange force between neighbours is much weaker than the force within the atomic shell of one ion. Although weak, the force is sufficient to cause ferromagnetism. Ferromagnetic materialsFerromagnetism is found in many insulators as well as metals. Chromium bromide (CrBr 3) is an insulator since chromium is trivalent and a bromine atom needs one electron to complete its outer shell. The trivalent chromium atoms each have a moment, and these align ferromagnetically below the Curie temperature of 37 K. Gadolinium chloride (GdCl 3; T c = 2.2 K) and europium oxide (EuO; T c = 77 K) are two other examples among many. Antiferromagnetic materials Zhang TL, Zhang B, Wang H, Jiang CB, Zhang ZH, Wang XQ, Zhang W. Low remanence temperature coefficient Sm 1− xEr x(Co, Fe, Cu, Zr) z magnets operating up to 400 °C. Rare Met. 2019. https://doi.org/10.1007/s12598-019-01223-4. Peng L, Yang QH, Zhang HW, Xu GL, Zhang M, Wang JD. Rare earth permanent magnets Sm 2(Co, Fe, Cu, Zr) 17 for high temperature applications. J Rare Earth. 2008;26(3):378.

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